chrysoprase and prase opal from haneti, central …...tutes for jade. dyed green agate and green...

NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY WINTER 2009 271

hrysoprase and prase opal are nickel-contain-ing green varieties of chalcedony and non-play-of-color (common) opal, respectively.

Both have been used as gem materials for thousandsof years. Chrysoprase was described as being the“most prized” type of chalcedony by Webster (1994,p. 233). Central Tanzania is an important contempo-rary source of both these gems. The Tanzanian mate-rials were first briefly described by Gübelin (1975, pp.76–78) and Schmetzer et al. (1976). These reportswere followed by a characterization of the green praseopal by Koivula and Fryer (1984), and a more detailedstudy of the chrysoprase by Kinnunen and Malisa(1990).

This article briefly describes the geology, mining,and gemological characteristics of high-qualityTanzanian chrysoprase and prase opal from theIyobo Mountain mine near Haneti (figure 1). Thismine is operated by Dimitri Mantheakis, who host-ed two of the authors (JES and BML) there in May2008. Although both materials have also been foundin several other parts of the world (table 1), Tanzaniais probably the most important source of chryso-prase after Australia.

BACKGROUNDThe green color of chrysoprase has long beenthought to be caused by one or more submicroscopicgreen nickel compounds (as Ni2+). While this ideahas been discussed for more than two centuries, theexact nature of these nickel compounds continues tobe debated (Natkaniec-Nowak et al., 1989). Someresearchers suggested it was a nickel oxide (possiblythe mineral bunsenite; see Heflik et al., 1989), butsubsequent spectroscopic and high-magnificationimaging studies disproved this idea (see Rossman,1994; Gawel et al., 1997). Most others have ascribedthe coloring agent to layered or framework hydratednickel silicates (such as kerolite) or nickel-contain-ing clay minerals (such as garnierite, lizardite, orsaponite; see Rossman, 1994, pp. 458–459; Nagase etal., 1997; Dyrek et al., 2001; Sachanbinski et al.,2001; Sojka et al., 2004).

The nickel compound(s) occur as tiny, colloidalparticles that are dispersed as inclusions throughoutthe host silica matrix. The presence of a smallamount of iron (as Fe3+) can modify the color ofchrysoprase to be more yellowish green, whereaslight scattering from microdefects or small particlesin the translucent material (the “Tyndall Effect”) hasbeen suggested as the cause of the more bluish green

C

James E. Shigley, Brendan M. Laurs, and Nathan D. Renfro

Commercial quantities of gem-quality chryso-prase and green prase opal (nickel-bearingchalcedony and common opal, respectively)have been recovered from altered serpentinitedeposits near Haneti, Tanzania. Material stud-ied for this report came from the largest mine,located on top of Iyobo Mountain, which hasbeen actively exploited for two decades.Although somewhat similar in color andappearance, chrysoprase and prase opal caneasily be distinguished from one another by dif-ferences in refractive index (~1.55 vs. ~1.45)and specific gravity (~2.60 vs. ~2.11). Chryso-prase makes up the vast majority of the output,and the mine shows good potential for contin-ued production.

NOTES & NEW TECHNIQUES

CHRYSOPRASE AND PRASE OPAL FROM

HANETI, CENTRAL TANZANIA

See end of article for About the Authors and Acknowledgments.GEMS & GEMOLOGY, Vol. 45, No. 4, pp. 271–279.© 2009 Gemological Institute of America

272 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY WINTER 2009

appearance seen in reflected light (Sachanbinski etal., 2001). Gawel et al. (1997) studied chrysoprasefrom several localities, and concluded that thosesamples with a lower degree of crystallinity tendedto have higher nickel contents and a more intensegreen color. Presumably, dispersed particles of nick-el-containing minerals also color the green praseopal from Tanzania, since the visible spectra of bothmaterials are similar (see Results section).Schmetzer et al. (1976) described this material asbeing opal-CT.

Because of their opaque-to-translucent greenappearance and ability to take a good polish, bothchrysoprase and prase opal have been used as substi-tutes for jade. Dyed green agate and green glass haveoccasionally been used to imitate chrysoprase(O’Donoghue, 2006). Also known is a chromium-colored green chalcedony found in Australia, Bolivia,and Zimbabwe (material from the last has beenreferred to as mtorolite; see Phillips and Brown,1989; Hyr`sl and Petrov, 1998; Hyr`sl, 1999; Willingand Stocklmayer, 2003). An opaque-to-semitranslu-cent yellow-green carving material from Australia,

sold under the name “lemon chrysoprase,” has beendescribed as a rock consisting of magnesite andquartz (Johnson and Koivula, 1996) or of both miner-als along with chrysoprase (Henn and Milisenda,1997). All these other materials have gemologicalproperties that allow them to be distinguished fromchrysoprase and prase opal.

OCCURRENCE AND DESCRIPTION OF THE TANZANIAN MATERIALLocation. Chrysoprase has been found on three adja-cent hills that lie ~12 km southeast of the village ofHaneti and 12 km north of the village of Itiso.Haneti is 75 km north of Dodoma along HighwayA104 (figure 2). The mine operated by Mr.Mantheakis is located near the top of the southern-most of the three hills (known as Iyobo Mountain;figure 3), at 5° 31.90′ S, 35° 59.33′ E and at an eleva-tion of 1,452 m. The area is accessible during the dryseason (which runs April through November) by adirt road from Dodoma to Haneti, and from there tothe mine site by a rough dirt track.

Figure 1. Tanzanian chryso-prase (left, cabochons rang-

ing from 3.72 to 18.42 ct)and prase opal (inset, 35.82ct) are commercially mined

from one deposit nearHaneti. All but the pieces of

rough were examined forthis study. Gift of Dimitri

Mantheakis; GIA Collectionnos. 37910–37914 (right)

and 37909 (inset). Photos byRobert Weldon.

TABLE 1. Other world sources of chrysoprase and prase opal.

Locality Geologic setting Reference

ChrysopraseMarlborough, Queensland, Weathered and laterized Brooks (1965), KroschAustralia serpentinites or other ultra- (1990), Jones (1992),

mafic rocks O'Brien (1997), Brown (2000), Downing (2007),Osmond and Baker (2009)

Warrawanda, Weathered and altered Nagase et al. (1997)Western Australia serpentinites associated

with granitesYerilla Station, Veins in weathered Jones (1994a,b), Western Australia siliceous ironstones Brown (2000)Niquelandia, Goiás, Brazil A galena mine (?) Kammerling et al.

(1990)Saxony, Germany Not reported Wittern (2001)Niigata Prefecture, Honshu, Not reported Mindat.org (2009)JapanSarykul Boldy, Kazakhstan Nickel deposit in Sachanbinski et al.

weathered serpentinite (2001)Ambatondrazaka, Not reported Behier (1963)MadagascarSzklary, Lower Silesia, Nickel deposit in Drzymala and SerkiesPoland weathered serpentinite (1973), Niskiewicz (1982),

Heflik et al. (1989), Sachanbinski et al. (2001)

Southern Urals, Russia Not reported Mikhailov (2000)Tulare County, California, Not reported O'Donoghue (1995)United StatesVarious sites in Arizona, Not reported Mindat.org (2009)California, Colorado, Massachusetts, New York, North Carolina, Oregon, Rhode Island, and Vermont

Prase opalSouth Bohemia, Not reported Duda et al. (1991)Czech Republic Silesia, Poland Not reported Drzymala and Serkies

(1973), Webster (1994)Southern Urals, Russia Small vein at a “chrome Mikhailov (2000)

mine”Napa County, California, Not reported “Prase opal in California,”United States 1936

Geology. According to Kinnunen and Malisa (1990),rocks in the area of the Haneti deposit are of Archeanage and consist of metamorphosed ultramafics thatform the three chrysoprase-bearing hills. These arealigned in a northwesterly direction, which marks theorientation of regional fracture/shear zones. Theydescribed the dominant rock type hosting the chryso-prase as a silicified and ferruginized (silica- and iron-altered) serpentinite.

KENYA

TANZANIA

Arusha

Haneti

Dodoma

Chrysoprase andprase opal mine

Morogoro Dar esSalaam

Moshi

Mombasa

A104

Zanzibar

Itiso

Tanga

INDIANOCEAN

A104

B129

A14

B1

B127A14

100 miles0

100 km0

Figure 2. The Haneti chrysoprase and prase opalmine is located in central Tanzania, betweenDodoma and Arusha.

TANZANIA

AFRICA

Both the chrysoprase and prase opal occur in dis-continuous veins up to several centimeters thickand several meters long in the weathered host rock.According to Mr. Mantheakis, the chrysoprase veinsappear to be more abundant on the western side andupper portions of Iyobo Mountain; some areas havealmost no such mineralization. Within a vein andbetween different veins, the material can vary fromhigh-quality translucent to low-quality opaque. Thebest chrysoprase occurs in those areas of a vein thatare surrounded by red clay or soil, while poorer-qual-ity material is found where the vein is enclosed byyellowish material. The best-quality prase opal isfound in veins within very brittle host rock.

Mining. Although at one time some Ni-rich magne-site veins were prospected in the region, the onlymining in the Haneti area has been for chrysoprase-

NEED TO KNOW• Chrysoprase and green prase opal are nickel-

bearing green chalcedony and common opal, respectively.

• They have been mined from Iyobo Mountain near Haneti, Tanzania, for more than two decades.

• They are found in veins hosted by altered serpentinite.

• Typically, 7–10 tonnes of “mine-run” chrysopraseis produced annually (3% suitable for cabochons).

• In the past decade, ~1.5 tonnes of “mine-run” prase opal was produced.


and prase opal for both gem and ornamental purpos-es (Kinnunen and Malisa, 1990). According to Mr.Mantheakis, chrysoprase was first found there in theearly 1960s, and limited production took placebefore the concession was nationalized by the gov-ernment in 1973, after which the mine lay dormant.Mr. Mantheakis, a Tanzanian citizen, reclaimed themine in 1986, and in 1997 was given full miningrights on the property.


Figure 3. The mine workings are situated on top ofIyobo Mountain and are marked by a row of tailings.

Photo by B. M. Laurs.

Mining is currently carried out in an open pit(figure 4) at the top of Iyobo Mountain. As many as35 local people are employed during the dry season.So far, 28 pits have been mined on the mountain,ranging from ~20 to ~250 m2. Mechanized equip-ment is used to expose areas containing the gem-bearing veins, which are then excavated with handtools (figure 5). The miners follow the veins untilthey pinch out (figure 6), and the chrysoprase andprase opal are collected by hand (figure 7) and placedinto bags for transport to a sorting facility on site.

The chrysoprase production is highly variable,ranging from as little as 100 kg to as much as 1,000kg per month; the typical annual production is 7–10tonnes. These quantities are for “mine-run” rough,which includes some chrysoprase in matrix. Aftersorting and trimming, ~20–30% of the chrysopraseis marketable, but only 3–5% is of high quality (i.e.,translucent, with even, intense color, and suitable forcutting attractive cabochons). The prase opal is muchless abundant: In the past decade it has been recov-ered from only two areas of the mine, which yieldeda total of ~1.5 tonnes of opal in matrix.

The better-quality material is sent to Dar esSalaam for trimming and polishing into cabochons.Some of the lower-quality rough is worked intobeads. The material is sold through wholesale andretail outlets in Dar es Salaam, as well as throughinternational trade shows.

Figure 4. Open-pitmining for chrysopraseand prase opal is performed using anexcavator and haulingvehicle. Photo by B. M. Laurs.

Figure 5. Shovels, pry bars, and picksare used to followthe gem-bearingveins. Photo by B. M. Laurs.

Mr. Mantheakis (pers. comm., 2008) reported thathis mining plan is sensitive to the local environ-ment. He places roads to preserve as many of thelarge trees as possible, and most of the pits are buriedas mining proceeds. He estimates that 80% of thedeposit remains to be mined.

MATERIALS AND METHODSFor this study, we examined nine chrysoprase sam-ples (five cabochons [3.72–18.42 ct] and four pre-formed or rough pieces weighing up to 8.6 g), andfive prase opal samples (one 35.82 ct cabochon andfour pre-formed or rough pieces weighing up to 7 g).All are representative of the better-quality materialbeing produced from Mr. Mantheakis’s mine. Wedocumented all the samples with a refractometer,gemological microscope, polariscope, desk-modelspectroscope, Chelsea color filter, and standard ultra-violet (UV) lamps.

All the samples were also analyzed by each of thefollowing methods. Qualitative chemical analyseswere performed using a Thermo ARL Quant-X ener-gy-dispersive X-ray fluorescence (EDXRF) systemwith multiple filter, voltage, and current settingsappropriate to the elements of interest. Ultraviolet-visible–near infrared (UV-Vis-NIR) absorption spec-tra were recorded over an interval of 250–2500 nmwith a Perkin Elmer Lambda 950 spectrometer,using a slit width of 2 nm and a data sampling inter-val of 0.5 mm. (Only the 400–700 nm region isshown in the results, since this is the region of great-est interest for investigating the origin of color.)Infrared spectra were recorded over the 6000–400cm−1 range with a Thermo Nicolet Magna-IR 760spectrometer, using a 6× beam condenser at a resolu-

tion of 4 cm−1 and 128 scans per sample to improvethe signal-to-noise ratio. We obtained Raman spectrafor both materials using a Renishaw InVia Ramanmicroscope with 514.5 nm laser excitation.

Figure 6. This vein at the chrysoprase/prase opalmine ranges from light green to deep green towardwhere it pinches out. Photo by B. M. Laurs.



RESULTS AND DISCUSSIONGemological Characteristics. Samples of both mate-rials were light to medium-dark slightly bluishgreen and opaque to translucent; all displayed a vit-reous polish luster. In general, the prase opal wasslightly more translucent than the chrysoprase. Thecolor distribution in both materials appeared uni-form or very slightly blotchy. In addition, the praseopal displayed small, dark, granular brownish areasalong fractures that Raman analysis indicated weremost likely goethite; each of these areas tended to besurrounded by a small halo where the adjacent praseopal was yellower (figure 8). The observation thatthe outline of the halos followed the shape of thedark areas suggests that iron from these inclusionspermeated the surrounding prase opal.

The RI and SG values of all tested samples fellwithin the following ranges, which readily separatethe two materials from the Haneti locality:

Chrysoprase RI 1.549 (±0.002)SG 2.57–2.63

Prase opal RI 1.455 (±0.004)SG 2.09–2.13

Kinnunen and Malisa (1990) reported similar RIand SG values for the chrysoprase from the Hanetiarea. These RI values are slightly above those ofchalcedony in general, but the SG ranges are consis-tent with those reported for chrysoprase in the litera-ture (Webster, 1994, pp. 252–253). We could not findproperties for prase opal in the literature, but the RIand SG values of our samples fell within the rangesgiven for opal in general by O’Donoghue (2006).

All samples were inert to both long- and short-wave UV radiation. They exhibited an aggregatereaction when viewed with a polariscope; this reac-tion was more pronounced for the prase opal.Neither material displayed any reaction whenviewed with a Chelsea color filter (i.e., all samplesremained green). The desk-model spectroscoperevealed broad absorption below ~450 nm and above~650 nm in both materials. These two regions ofabsorption were more intense, and the interveningregion of light transmission narrowed in wavelengthrange, for the more saturated green samples of bothmaterials. No sharp absorption lines were observedin the spectra of either material.

The gemological properties of both the chryso-prase and prase opal were generally consistent withreports in the literature for samples from the Hanetiarea and elsewhere. Kinnunen and Malisa (1990)characterized the chrysoprase as displaying a distinctmicrotexture consisting of disordered silica spherules(~40–80 μm diameter), formed of concentric layers ofquartz, chalcedony, and opal-A, within a silica

Figure 7. The veins of chrysoprase (shown here)and prase opal typically must be trimmed fromthe host matrix. Photo by B. M. Laurs.

Figure 8. This dark inclusion (probably goethite)is surrounded by a yellowish green halo. Itappears that iron from the inclusion permeatedthe surrounding prase opal. Photomicrograph byJ. E. Shigley; magnified 15×.

WAVENUMBER (nm)

AB

SOR

PTIO

N C

OEF

FIC

IEN

T

VISIBLE-RANGE ABSORPTION SPECTRA

700550 650600500450400

5 cm-1

0

Chrysoprase

Prase opal

WAVENUMBER (cm-1)

55006000 5000 4500 4000

AB

SOR

BA

NC

E

1

05245

45174326

4300

Chrysoprase

Prase opal

NEAR-IR ABSORPTION SPECTRA

500 nm and a broad peak centered near 650 nm (fig-ure 9), corresponding to the absorption patterns seenwith the spectroscope (see also Schmetzer et al.,1976; Sachanbinski et al., 2001). This is consistentwith the spectrum for chrysoprase from TulareCounty, California (Rossman, 2009). The region ofabsorption above 650 nm has been attributed to Ni2+

in octahedral coordination in a silicate mineral(Rossman, 1994, pp. 458–459; there is no indicationthat this element substitutes for either silicon oroxygen in either the chrysoprase or the prase opal).

The near-infrared spectra were similar for bothmaterials (figure 10), with a broad band at ~5245 cm−1

and a group of features at 4517, 4326, and 4300 cm−1

due to the presence of water molecules or hydroxylgroups in both materials (similar spectra for opals areillustrated in Langer and Flörke, 1974).

In contrast, the Raman spectra of the two materi-als were very different from one another (figure 11).In the chrysoprase, there were sharp Raman peaks at1160, 807, 463 (much more intense than the otherpeaks), 398, 354, and 264 cm−1. All of these featureswere reported previously in a study of the Ramanspectra of microcrystalline silica (including chal-cedony) by Kingma and Hemley (1994). The Ramanspectra of the prase opal exhibited peaks at 783, 671,and a broader feature at 325 cm−1. Similar features at~800–780 and ~325 cm−1 were reported in opals byOstrooumov et al. (1999) and by Smallwood (2000).

Figure 9. These visible-range spectra are for repre-sentative polished slabs of similar thickness of thechrysoprase (path length 1.86 mm) and prase opal(path length 1.76 mm). Both spectra exhibitregions of absorption below 500 nm and above 600nm. The inset photo (by Robert Weldon) shows thesimilar color of both materials, as well as thegreater transparency of the prase opal, on the right.

Figure 10. These representative infrared absorptionspectra of the chrysoprase and prase opal display aband at ~5245 cm−1 (more intense in the prase opal)and a group of similar features at 4517, 4326, and4300 cm−1.


groundmass. In contrast, they found that the silicaspherules in the Tanzanian prase opal were an orderof magnitude smaller (~5–6 μm). They described thechrysoprase as also containing “cloudy color distribu-tions, clear chalcedony veinlets, whitish dots (micro-cavities), small fluid inclusions in bypyramidalquartz, and brownish inclusions of host rock” (p. 162)based on microscopic observations (10×–60× magni-fication and various lighting conditions). They inter-preted the lack of a vapor phase in the liquid inclu-sions as implying that the chrysoprase formed at avery low temperature and was deposited by repeatedevaporation of Si-rich hydrothermal solutions or sur-face waters along open fractures in the altered serpen-tinite host rock.

Chemical Analysis. Our EDXRF analyses indicatedmajor amounts of Si and minor Ni, Zn, and Fe inboth materials. Using atomic absorption spec-troscopy, Kinnunen and Malisa (1990) measured 0.55wt.% Ni in the chrysoprase, along with traceamounts of other elements (including the transitionmetals Co, Zn, Fe, Mn, and Cu at values up to 120ppm, and rare-earth elements at 1 ppm or less).

Spectroscopy. The visible-range spectra of bothmaterials showed increasing absorption below about


RAMAN SHIFT (cm-1)

RAMAN SPECTRA

2001400 100012002000 1800 1600 800 400600

783671

325463

264354

3988071160Chrysoprase

Prase opal

INTE

NSI

TY

CONCLUSIONJewelry-quality chrysoprase and prase opal (figure 12)are mined from altered serpentinite near Haneti,Tanzania. The gemological properties of both mate-rials are generally consistent with previous reportsfor chrysoprase and opal, and we did not find any fea-tures that distinguish them from the same materialsfrom other localities. Chrysoprase can be separatedfrom chrome chalcedony by the latter’s yellow UVfluorescence, red Chelsea color filter reaction, andsharp absorption line at 684 nm seen with a spectro-scope (Hyr`sl, 1999). Although similar in color andvisual appearance, chrysoprase and prase opal fromthe Haneti area can be readily distinguished fromone another on the basis of RI and SG values, as wellas by their Raman spectra. Field observations of themining site on Iyobo Mountain suggest good poten-tial for future production.

ABOUT THE AUTHORSDr. Shigley is distinguished research fellow, and Mr. Renfrois a staff gemologist, at the GIA Laboratory in Carlsbad,California. Mr. Laurs is editor of Gems & Gemology at GIAin Carlsbad.

ACKNOWLEDGMENTSThe two senior authors thank Dimitri Mantheakis of RuvuGemstone Mining Ltd., Dar es Salaam, Tanzania, for pro-viding information, donating samples, and hosting them athis mining operation near Haneti.

Figure 12. These Tanzanian prase opal samplesconsist of a 10.22 ct cabochon and a 5.15 ctfaceted stone, together with a vein of roughmaterial (GIA Collection nos. 32590, 32589,and 32587, respectively; photo by RobertWeldon). Tanzanian chrysoprase has been setinto a variety of jewelry styles, as shown bythe earrings in the inset (~9 ct total weight;courtesy of Dimitri Mantheakis).

Figure 11. The Raman spectra for the chrysopraseand prase opal differ significantly.

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chrysoprase and prase opal from haneti, central …...tutes for jade. dyed green agate and green...

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